In vehicles with a power supply, when an insulation failure or the like occurs between a bus line either on a plus side or minus side of the power supply and a vehicle body, there is a risk that a so-called ground fault resulting in a leakage of power supply current to the vehicle body may occur. Thus, an insulation resistance detector is provided for detecting an insulation resistance between a DC power supply and a vehicle body. The insulation resistance detector is often referred to as an “insulation resistance drop detector” because the insulation resistance detector is used to detect a drop in insulation resistance. Further, when a failure occurs in the insulation resistance drop detector itself, it becomes impossible to detect the insulation resistance drop. Therefore, a failure of the insulation resistance drop detector itself is detected.
For example, Japanese patent publication JP 2005-114496 A discloses an insulation resistance drop detector for detecting a drop in insulation resistance of an insulating resistor Ri between a DC power supply and a vehicle body (ground). The insulation resistance drop detector is configured to include a detecting resistor Rd connected in series with the insulating resistor Ri, a coupling capacitor disposed between the insulating resistor Ri and the detecting resistor Rd, a pulse oscillating circuit for applying a pulse signal to a series circuit including the insulating resistor Ri, the coupling capacitor, and the detecting resistor Rd, a filter for removing a signal component having a predetermined frequency from a voltage at a junction between the detecting resistor and the coupling capacitor, and a detector for detecting a drop in insulation resistance based on an output from the filter.
It should be noted that, in the above-noted patent document, a drop in insulation resistance is detected by observing changes of the frequency characteristic of the filter output while changing frequency of the pulse oscillating circuit. More specifically, when the coupling capacitor functions properly, the filter output shows a certain frequency characteristic. On the contrary, when the coupling capacitor is short-circuited, the filter output stays at a low level regardless of the frequency. On the other hand, when the coupling capacitor is opened, the filter output stays at a high level even with an increased frequency. The above patent document describes that these differences enable the detection of a failure of the insulation resistance drop detector.
Another patent document, Japanese patent publication JP 2004-354247A, points out that, with the prior art, it is difficult to detect an electrical leakage of a DC power supply to a vehicle body with a change of a common mode potential which is a potential of a battery terminal with respect to a vehicle body. It is noted that a vehicle impedance changes depending on whether a relay, which is provided between a battery and an inverter, is turned ON or OFF. When the relay is ON, the common mode voltage changes to low, while when the relay is OFF, the common mode voltage changes to high. It is disclosed to provide means for generating a pseudo electrical leakage so that it is possible to determine that an electrical leakage detecting device functions properly when the pseudo electrical leakage is detected. It is disclosed further that when an opening or closing of the relay is detected, the generation and detection of the pseudo electrical leakage are retried to avoid receiving effect of the change of the common mode potential.